The present invention relates to a digital frequency discriminator intended for extracting from a carrier at a frequency of f.sub.o at least one low frequency sinusoidal signal, of frequency F, frequency modulating the said carrier with a modulating frequency excursion .DELTA.F, the values of f.sub.o and .DELTA.F being known.
The present invention further relates to the use of a discriminator for determining the phase known as the reference phase contained in the composite VOR signal, received by an on-board VOR-ILS equipment, by point by point restoration of the 30 Hz sinusoidal signal giving the reference phase.
Most of the frequency discriminators used up to the present time are analog discriminators. They are used in particular in radio navigation. For example, in a radio altimeter of the linear frequency modulated continuous wave type, it is a known technique to hold the subtractive beat frequency f.sub.b between the transmitted wave and the received reflected wave at a constant set frequency f.sub.bo of predetermined value by including such a discriminator in the feedback system which controls the slope of the modulation line of the transmitted signal. This discriminator which has the function of providing, according to a substantially linear characteristic, the difference between the frequency f.sub.b and the set frequency, is basically formed of two processing systems each of which receives the signal to be discriminated. Each processing system includes a band pass filter, centered on f.sub.bo -.delta.f (and centered on f.sub.bo +.delta.f) respectively), followed by an absolute value detector. The two signals thus obtained are subtracted from each other and the resultant signal is filtered through a low pass filter, An S-shaped characteristic amplitude-frequency curve is thus obtained whose central section, on either side of a phase shift equal to .pi./2, is substantially linear. From the work "Digital Signal Processing" by V. Capellini and A. G. Constantini, published by Elsevier Science Publishers B.V. (North Holland 1984), pages 632 to 637, the technique is known of digitally transposing the structure of the analog discriminator described above. The six components which form this structure are thus produced digitally, which obtains certain advantages such as ease of adjustment during mass production for example. In this way the previously mentioned S-shaped characteristic curve is again obtained. This digital discriminator has disadvantages: its linearity is limited. Also, the filtering thus achieved requires a large computing capacity and the result obtained, even if it is acceptable for a continuous frequency spectrum having a leading edge which it is sought to discriminate as is the case for the beat signal of a radio altimeter, has a lower performance when the discriminator of a single frequency line is desired, for example by demodulation of a VOR-ILS signal.